While researches in biological fields are looking to quantum dots to replace organic fluorescent dyes, quantum dots also hold promise for use in electronic devices. Research is ongoing into incorporating quantum dots into photovoltaics, solid-state lighting (mainly as quantum dot phosphors), electroluminescent displays, and quantum computing devices. Semiconductor light emitting diode (LED) devices have been made since the early 1960s and currently are manufactured for usage in a wide range of consumer and commercial applications. The layers including the LEDs are based on crystalline semiconductor materials that require ultra-high vacuum techniques for their growth, such as, metal organic chemical vapor deposition. In addition, the layers typically need to be grown on nearly lattice-matched substrates in order to form defect-free layers. These crystalline-based inorganic LEDs have the advantages of high brightness (due to layers with high conductivities), long lifetimes, good environmental stability, and good external quantum efficiencies. The usage of crystalline semiconductor layers that results in all of these advantages, also leads to a number of disadvantages including high manufacturing costs, difficulty in combining multi-color output from the same chip, and the need for high cost and rigid substrates.
Since the mid 1980s, LED displays have been brought out into the marketplace and there has been great improvements in device lifetime, efficiency, and brightness. For example, devices containing phosphorescent emitters have external quantum efficiencies as high as 19%; whereas, device lifetimes are routinely reported at many tens of thousands of hours. In comparison to crystalline-based inorganic LEDs, OLEDs have much reduced brightness (mainly due to small carrier mobilities), shorter lifetimes, and require expensive encapsulation for device operation. On the other hand, OLEDs enjoy the benefits of potentially lower manufacturing cost, the ability to emit multi-colors from the same device, and the promise of flexible displays if the encapsulation issues can be resolved.
To improve the performance of OLEDs, quantum dots were introduced in to the emitter layers to enhance the color gamut of the device and reduce manufacturing costs. Because of problems, such as, aggregation of the quantum dots in the emitter layer, the efficiency of these devices was rather low in comparison with typical OLED devices. The efficiency was even poorer when a neat film of quantum dots was used as the emitter layer. Regardless of any future improvements in efficiency, these hybrid devices still suffer from all of the drawbacks associated with pure OLED devices.
Recently, all-inorganic LEDs have been constructed by, for example, sandwiching a monolayer thick core/shell CdSe/ZnS quantum dot layer between vacuum deposited n- and p-GaN layers. However, such devices exhibit poor external quantum efficiency of 0.001 to 0.01% because of organic ligands of trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP) insulators that result in poor electron and hole injection into the quantum dots. In addition, the structure is costly to manufacture, due to electron and hole semiconducting layers grown by high vacuum techniques, and sapphire substrates. Accordingly, it would be highly beneficial to construct an all inorganic LED based on quantum dot emitters which was formed by low cost deposition techniques and whose individual layers showed good conductivity performance. The resulting LED would combine many of the desired attributes of crystalline LEDs with organic LEDs.